1. Field of the Invention
The present invention relates to an optical communication system and in particular to a passive optical network system.
2. Description of the Related Art
In order to construct a subscriber network from a central office to buildings and homes, various network constructions such as x-Digital Subscriber Line (xDSL), Hybrid Fiber Coax (HFC), Fiber To The Building (FTTB), Fiber To The Curb (FTTC), Fiber To The Home (FTTH) and schemes for enhancing them have been proposed.
Such FTTx (i.e., FTTB, FTTC and FTTH) can be implemented by active FTTx based on an Active Optical Network (AON) construction, or implemented by passive FTTx based on a Passive Optical Network (PON) construction. Passive optical networks have been proposed as an economic scheme for implementing an optical subscriber network for the future, due to the fact that it comprises a point-to-multipoint topology based on passive units.
The passive optical network is a subscriber network construction that connects one Optical Line Termination (OLT) to a plurality of Optical Network Units (ONU) by using a 1×N passive optical distributor, thereby forming a tree-structured distribution topology.
Currently, internet services including digital broadcast services and communication services such as Video On Demand (VOD) services are independently provided. Systems for digital broadcast services are designed for use in a broadcast network such as a Hybrid Fiber Coax (HFC) broadcast network or a Direct Broadast Satellite (DBS) network. This separation of services is the result of the complicated and dedicated hardware apparatus that make up these respective systems. For example, a head-end and an optical line termination should be operated separately in order to provide broadcast and communication services, and sub-carrier analog transmission should be executed in order to transmit data to an optical network unit, in which case a complicated modulation mode is employed for increasing transmission efficiency. In addition, because in-house connecting networks are constructed using coaxial cable type, the transmission bandwidth is limited to about 900 MHz. Furthermore, with regard to broadcast networks, subscriber bandwidth utilization is unstructured and there are also associated limitations in transmission distance because a mode is employed in which all broadcast channels are transmitted to all of the subscribers. Although cable-modems are employed for use with the internet service receiving classes, there is a problem in that it is impossible to provide efficient bandwidth for increasing subscribers due to the fact that limited services are provided to the subscribers.
FIG. 1 shows a structure of a digital broadcast service system according to the prior art. A head-end 130 and an optical line termination 160 are shown. Digital broadcast signals are received at the head-end 130 from either a satellite antenna 110 or a Digital Media Center (DMC) 120. The digital broadcast signals may be in the form of an MPEG2 multi-program transport stream, which consists of a plurality of individual MPEG2 transport streams. The head-end 130 comprises a transport stream demultiplexer 140 and a plurality of local processors 150.
The transport stream demultiplexer 140 demultiplexes the MPEG2 multi-program transport stream into a plurality of MPEG2 transport streams.
The local processors 150 remultiplex the MPEG2 transport streams for the purpose of performing encoding processing, customer management processing or the like.
The optical line termination 160 consists of a plurality of quadrature amplitude modulators (QAM) 170, a plurality of frequency converters 180, a signal combiner 190 and an electric/optical (E/O) converter 200.
The quadrature amplitude modulators 170 change the phase and amplitude of carrier, thereby enabling high-speed digital transmission. The frequency converters 180 convert the frequency of an inputted signal into an intermediate frequency (IF). The RF combiner 190 multiplexes and thus outputs a plurality of inputted signals as one broadcast signal. The E/O converter 200 E/O converts and transmits the broadcast signals through an optical fiber. It is possible to employ a laser diode as the E/O converter 200, which outputs light having a predetermined wavelength.
The broadcast signals, which have been subjected to the E/O conversion, are provided to a plurality of optical network units (not shown) connected to the optical line termination 160 through the optical fiber 210.
FIG. 2 shows frequency band allocation of an HFC network of the prior art. As shown in the drawing, it will be appreciated that the transmission band width is limited to 825 MHz; because the frequency bandwidth for V OD, broadcast service, etc. is narrow, because there is difficulty in providing real VOD services, high-speed internet services or the like.
As can be seen, the following problems exist in providing communication services such as digital broadcast services, VOD services, internet services or the like in accordance with the prior art network.
i) Because digital broadcast services and communication services are individually and separately provided, use and management of resources are inefficient.
ii) The digital broadcast is adapted to an HFC network. Therefore, it is necessary to employ a complicated modulation mode such as 64-QAM and 256-QAMin order to enhance the transmission efficiency of the broadcast signals, which are sent from a satellite or a digital media center.
iii) The head-end and the optical line termination should be separately operated, and the sub-carrier analog optical transmission should be performed in order to transmit data to an optical network unit. However, because in-house connection networks are in the form of coaxial cable, there is a problem in that the transmission bandwidth is not more than about 900 MHz.
iv) Although there is a cable-modem type in communication service receiving classes, no efficient bandwidth can be provided for increasing subscribers because limited services are allocated to subscribers.